1. Field of the Invention
The present invention relates to method and system for detecting the existence of vehicles, passage of vehicles, and the like, and for communicating between the vehicle side and the ground side. Such method and system are applied to the following systems: namely, the traffic control system which counts the number of vehicles passed on the basis of the detection of the vehicles, thereby smoothing the flow of vehicles; the system which detects the vehicles which run a toll road and automatically collects the tolls; the system which detects the vehicles entering a parking lot and automatically collects the parking fees; the system which informs the jam situation and the like of an express highway or other roads to each vehicle; the system which detects the running positions of the special vehicles such as bus, patrol car, taxi, unmanned conveying truck which runs in a warehouse, and the like and at the same time communicates between these vehicles and the control center; and the like.
2. Description of Prior Art and Prior Japanese Background Application
As typical one of conventional vehicle detecting systems, there has been known a system including an almost square loop coil buried under the road surface. A high frequency exciting current flows through the loop coil. When the vehicle passes above the loop coil, the inductance of the coil changes, so that the value of the current also varies. The passage of the vehicle can be recognized by detecting the change in this current.
As background to this invention has already proposed the system using such a loop coil in which the communication is performed between the vehicles passing above the loop coil and the control center on the ground side (Japanese Patent Application No. 60-41678). According to this '678 system, in the case of transmitting data to the vehicle side from a control unit installed at a predetermined location (hereinafter, this control unit or the like is referred to as a ground side), a high frequency current which is allowed to flow through the loop coil is modulated by the data to be transmitted, and the modulated signal is received by a receiver equipped in the vehicle. In the case of transmitting data from the vehicle side to the ground side, a signal having a frequency different from that of the foregoing high frequency current is transmitted from a transmitter provided for the vehicle. At this time, the loop coil functions as a kind of antenna and receives the signal from the vehicle. Therefore, by demodulating this signal, the vehicle data can be obtained on the ground side.
However, such a communicating system has the following problems.
One loop coil is commonly used to transmit the high frequency signal from the ground side to the vehicle side, and to receive the signal transmitted from the vehicle side and take out the vehicle detection signal. Therefore, switching means is needed and the switching operation of the switching means must be controlled, causing the constitution to become complicated.
After completion of the detection of the vehicle, data is transmitted to the vehicle and then the data from the vehicle is received. In this manner, together with the changeover of the switching means, these operations must be time-sharingly executed. Consequently, in the case where a plurality of vehicles pass the roadway at a fairly short distance between vehicles, there is the fear of occurrence of the malfunction of the system.
Further, the vehicle detecting system using the loop coil has the following problems.
In general, the loop coil has the size of about 2 m.times.2 m and the road must be dug up over a wide range to bury such a large loop coil into the road. Such a burying construction is a large-scaled, so that the construction expenses increase and much labors are needed for the construction.
The loop coil buried under the road surface is frequently subjected to the loads in association with the passages of vehicles, so that the accident of disconnection of the coil is likely to occur. The occurrence of the accident of the disconnection disenables the detection of vehicles.